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Why some sharks glow in the dark

US researchers have discovered a family of small-molecule metabolites that explains how and why some sharks can glow in the dark, and only other sharks can see it.

It works in quite a different way – and for a very different purpose – to the mechanism used by other marine creatures, and as an added bonus might also help them fight microbial infections.

“After we first reported that swell sharks were biofluorescent, my collaborators and I decided to dive deeper into this topic,” says David Gruber from the City University of New York. “We wanted to learn more about what their biofluorescence might mean to them.”

What they learnt is reported in paper in the journal iScience.

Gruber and Jason Crawford, from Yale University, studied swell sharks and chain catsharks and noticed that their skins had light and dark tones. They extracted chemicals from the skins and found a type of fluorescent molecule that was only present in the light skin.

These types of small-molecule metabolites are known to be fluorescent and activate pathways similar to those that, in other vertebrates, play a role in the central nervous system and immune system.

But in the sharks, the novel variants account for the biophysical and spectral properties of their lighter skin. This mechanism is different from animals in the upper ocean, such as jellyfish and corals, that commonly use green fluorescent proteins as mechanisms to transform blue light into other colors, Gruber says.

“It’s a completely different system for them to see each other that other animals cannot necessarily tap into,” Crawford adds. “They have a completely different view of the world that they’re in because of these biofluorescent properties that their skin exhibits and that their eyes can detect.”

The researchers also found that these biofluorescent molecules display antimicrobial properties.

“These catsharks live on the ocean bottom, yet we don’t see any biofouling or growth, so this could help explain yet another amazing feature of shark skin,” says Gruber.

“This study opens new questions related to potential function of biofluorescence in central nervous system signalling, resilience to microbial infections, and photoprotection.”